It was over 1900 years ago that the ancestors of today's Thearaddans began to move into Sacawisha. These people came from the open plains of Wistresh, to the west. The first Thearaddans came as mercenary soldiers, fighting for various petty lords of the Wesstua; that is, the previous human occupants of Sacawisha.
The Wesstua were a short, dark-skinned people, relatives to the Amagareshi people (the region southeast of Sacawisha). At that point in history, they were divided into a large number of small princedoms, barely more than city-states. These princes fought each other very freely—thus the need for mercenaries.
When those first mercenaries saw the lands along what is now the River Nelegorant, in modern-day Animona, they were amazed at the gentleness and fertility of the region. The plains of Wistresh are high and dry, studded with canyons and hills of bare rock, good only for raising cattle. These mercenary cavalry reported back to their people that they had found paradise.
It remained only for a clever leader to arise among the proto-Thearaddans. Brutal and capable, a man named Kormonan (who ascended to Interceder status after his death) organized the first onslaught. After several bloody years, he carved out a new realm in the upper reaches of the River Nelegorant. A flood of Thearaddan settlers poured into this new land. The surviving Wesstuans were expelled or assimilated. After fifteen years, no one, entering the new nation for the first time, would have even known that the Wesstua had once lived and ruled there.
Inevitably, the flow of Thearaddan settlers produced population pressure. Just as inevitably, now that the weakness of the Wesstua had been exposed, the immigrants set out to conquer new lands of their own. The battles went on for generations, the Thearaddans slowly expanding, first south along the Nelegorant, and eventually east toward the River Halavo. A series of campaigns, fought in the Jokogan Mountains, cleared the entire northwest of Wesstuans. This was about 1600 years ago.
There was a temporary setback, when the Wesstua got hold of a powerful (and evil) magical artifact, but the Thearaddan gods, acting through Interceders, brought heroes [the PCs of the campaign, as it happened] from the future to defeat the Crystal Mace and the madman wielding it.
After this, the Thearaddan invasion assumed the invincibility of an incoming tide. A further 300 years saw the invaders in control of all of today's Sacawisha, as well as the area now engulfed by the forest of Lorsana. There were only two holdouts. In the Neraifi Canduina, a small range of hills near present-day Chelmarrond, die-hard Wesstuans held out for another 150 years before being exterminated. Along the River Elave, in the town of Elle-Ruustu, an enclave of Wesstuans, who kept their racial identity, lives on to this day.